In business equipment with a need to deal with molten salt under a high temperature condition equal to or above a melting point, generally, it is likely that there is a phenomenon in which vapor, mist and fine particles, generated from such molten salt, result in the clogging of a filter, contamination of product gas and the clogging of a delivery conduit. Thus, an exceedingly large number of matters need to be considered for equipment structures and processing steps.
In production of, for instance, titanium, a process has adopted a step of obtaining titanium on reduction of titanium tetrachloride with magnesium metal. In such a process, used magnesium is collected as magnesium chloride and magnesium metal can be obtained again by conducting electrolysis of resulting magnesium chloride as molten salt, thereby achieving recycle of magnesium metal.
In molten salt electrolysis of such magnesium chloride, magnesium chloride has a melting point of 714° C. while magnesium has a melting point of about 650° C. and, hence, magnesium can be extracted in molten metal provided that a temperature of molten salt is 650° C. or more. In order to perform electrolysis at a temperature equal to or above 650° C. but at a temperature as low as possible, alkaline metal has been added to magnesium chloride to be molten in an eutectic state under which electrolysis is conducted.
Reasons why molten salt electrolysis of magnesium chloride is performed at such a lower temperature is because of not only a need to achieve energy saving but also a need to minimize the occurrence of clogging of a gas extraction delivery conduit in the presence of vapor, mist and fume, contamination of the device, a loss of magnesium chloride or the like, caused when molten liquid of magnesium chloride is kept at a high temperature. In actual practice, however, it is a present situation that even if an attempt is made to incorporate such additive with a view to lowering the melting point for electrolysis, the occurrence of the clogging in a delivery conduit portion or the like cannot be reliably suppressed due to magnesium chloride.
In production of silicon, further, zinc metal is used in reduction of silicon tetrachloride and, hence, there is a need to electrolyze molten salt of zinc chloride generated as a by-product. In such a case, zinc has a melting point of 413° C. whereas zinc chloride has a melting point of about 280° C. In order for zinc to be extracted in a molten state, therefore, there is a need for electrolysis to be conducted at a temperature as high as 413° C. or more that is higher in temperature than the melting point of zinc chloride by 100° C. or more. That is, it is more difficult in a present situation to avoid the occurrence of clogging of the gas extraction delivery conduit due to vapor, mist and fine particles generated from molten salt.
Furthermore, such a phenomenon with the occurrence of clogging of the gas extraction delivery conduit due to vapor, mist and fine particles generated from such molten salt can be seen not only in production of titanium or silicon but also in overall process of dealing with molten salt at high temperatures. It can be said that such a phenomenon is one of important issues inherent to such a process in nature to be studied.
Under such circumstances, Patent Publication 1 proposes an electrolyte cell in which an air space is provided in an upper area of electrolytic liquid for storing resulting electrolytic product gas with a view to returning resulting vapor and mist to electrolytic liquid with only electrolytic gas being delivered through a gas extraction delivery conduit.